I dunno. The necro always struck me as fighting to prove he was the true master of evil or something. He also seemed to have a wicked sense of humor (from the few sound bites haha).

Though I suppose he did sort of fight for good using dark powers. O_o

Although he liked the smell of death, but angrily refered to the stench of demons. He also sounded upset after killing Duriel because Diablo escaped, and that terror and destruction were roaming free. Tyrael was friendly with him too.

The Necromancer wasn't out to prove anything, nor was he aligned with darkness or evil. He simply utilized the Prime Energies of Sanctuary to manipulate cadavers and spirits in a bid to rid the world of extra-planar threats -- namely, that of Hell. He and the other Priests of Rathma fought alongside Tyrael only because he represented a tool to better aid them against Diablo's legions.

Be it through hallowed grounds or lands of sorrow
The Forger's wake is bereft and fallow

Is the residuum worth the cost of destruction and maiming;
Or is the shaping a culling and excercise in taming?

The road's goal is the dark Origin of Being
But be wary through what thickets it winds.

-Excerpt from the Litany of Residuum;
As Translated by He Who Brings Order